Concrete cover
Encyclopedia
Concrete cover, in reinforced concrete
, is the least distance between the surface of embedded reinforcement
and the outer surface of the concrete
(ACI
130). The concrete cover depth can be measured with a cover meter
.
The premature failure of corroded steel reinforcements and the expansion of the iron corrosion products around the rebars are amongst the main causes of the concrete degradation. The carbon steel of rebars is protected from oxidation by atmospheric oxygen
by the high pH
of concrete interstitial water. Iron bar surface is passivated as long as the pH value is higher than 10.5. Fresh cement water has a pH of about 13.5 while evolved cement water pH ~ 12.5 is controlled by the dissolution of calcium hydroxide
(portlandite
). Carbon dioxide
present in the air slowly diffuses through the concrete cover over the rebar and progressively reacts with the alkaline hydroxides (KOH
, NaOH) and with calcium hydroxide leading to the carbonation of the hydrated cement paste. As a result, the pH of the cement drops and when its value is below 10.5 – 9.5, steel surface is no longer passivated and starts to corrode. A sufficient thickness of concrete cover is thus required in order to slow down the carbonation
process towards the rebar. The minimum concrete cover will depend on the environmental conditions encountered and must be thicker when the concrete is also exposed to moisture and chloride
(proximity to the sea, use of de-icing salt for bridges or roads, ...). A high quality concrete made with a low water-to-cement (w/c) ratio will have a lower porosity
and will be less permeable to water and to the ingress of corrosive species (dissolved oxygen, chloride, ...). A thicker cover or a more compact concrete will also reduce the diffusion of CO2 in the concrete, protecting it better from carbonation and maintaining a higher pH for a longer time period, increasing so the rebar service life.
also specify minimum cover requirements based on their respective local exposure conditions.
widths in flexural reinforced concrete members. Large crack-widths (greater than 0.3 mm) permit ingress of moisture and chemical attack to the concrete, resulting in possible corrosion of reinforcement and deterioration of concrete
. Therefore, thick covers defeat the very purpose for which it is provided. There is a need for judicious balance of cover depth and crack width requirements.
A possible economical solution for this paradox is the placing of a second layer of corrosion-resistant reinforcement like stainless steel
rebars or mesh
es or FRP
rebars in the concrete cover to distribute the cracks.
Reinforced concrete
Reinforced concrete is concrete in which reinforcement bars , reinforcement grids, plates or fibers have been incorporated to strengthen the concrete in tension. It was invented by French gardener Joseph Monier in 1849 and patented in 1867. The term Ferro Concrete refers only to concrete that is...
, is the least distance between the surface of embedded reinforcement
Reinforcement
Reinforcement is a term in operant conditioning and behavior analysis for the process of increasing the rate or probability of a behavior in the form of a "response" by the delivery or emergence of a stimulus Reinforcement is a term in operant conditioning and behavior analysis for the process of...
and the outer surface of the concrete
Concrete
Concrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...
(ACI
American Concrete Institute
The American Concrete Institute is a non-profit technical society and standard developing organization . ACI was founded in 1904 and its headquarters are currently located in Farmington Hills, Michigan, USA.- ACI History :1904-1929:...
130). The concrete cover depth can be measured with a cover meter
Cover meter
A cover meter is an instrument to locate rebars and measure the exact concrete cover. Rebar detectors are less sophisticated devices that can only locate metallic objects below the surface...
.
Purpose of provision of concrete cover
The concrete cover must have a minimum thickness for three main reasons:- to protect the steelSteelSteel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
reinforcement bars (rebars) from environmental effects to prevent their corrosionCorrosionCorrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen...
; - to protect the reinforcement bars from fire, and;
- to give reinforcing bars sufficient embedding to enable them to be stressed without slipping.
The premature failure of corroded steel reinforcements and the expansion of the iron corrosion products around the rebars are amongst the main causes of the concrete degradation. The carbon steel of rebars is protected from oxidation by atmospheric oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
by the high pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
of concrete interstitial water. Iron bar surface is passivated as long as the pH value is higher than 10.5. Fresh cement water has a pH of about 13.5 while evolved cement water pH ~ 12.5 is controlled by the dissolution of calcium hydroxide
Calcium hydroxide
Calcium hydroxide, traditionally called slaked lime, is an inorganic compound with the chemical formula Ca2. It is a colourless crystal or white powder and is obtained when calcium oxide is mixed, or "slaked" with water. It has many names including hydrated lime, builders lime, slack lime, cal, or...
(portlandite
Portlandite
Portlandite is a rare oxide mineral, the naturally occurring form of calcium hydroxide . It is the calcium analogue of brucite .-Occurrence:...
). Carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
present in the air slowly diffuses through the concrete cover over the rebar and progressively reacts with the alkaline hydroxides (KOH
KOH
-Chemistry:*Potassium hydroxide, formula "KOH"** KOH test, a procedure in which potassium hydroxide is used to dissolve skin and reveal fungal cells under the microscope** KOH titration, a procedure for determining a lubricant's acid level or TBN-Person's name:...
, NaOH) and with calcium hydroxide leading to the carbonation of the hydrated cement paste. As a result, the pH of the cement drops and when its value is below 10.5 – 9.5, steel surface is no longer passivated and starts to corrode. A sufficient thickness of concrete cover is thus required in order to slow down the carbonation
Carbonation
Carbonation is the process of dissolving carbon dioxide in water. The process usually involves carbon dioxide under high pressure. When the pressure is reduced, the carbon dioxide is released from the solution as small bubbles, which cause the solution to "fizz." This effect is seen in carbonated...
process towards the rebar. The minimum concrete cover will depend on the environmental conditions encountered and must be thicker when the concrete is also exposed to moisture and chloride
Chloride
The chloride ion is formed when the element chlorine, a halogen, picks up one electron to form an anion Cl−. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. The chloride ion, and its salts such as sodium chloride, are very soluble in water...
(proximity to the sea, use of de-icing salt for bridges or roads, ...). A high quality concrete made with a low water-to-cement (w/c) ratio will have a lower porosity
Porosity
Porosity or void fraction is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume, between 0–1, or as a percentage between 0–100%...
and will be less permeable to water and to the ingress of corrosive species (dissolved oxygen, chloride, ...). A thicker cover or a more compact concrete will also reduce the diffusion of CO2 in the concrete, protecting it better from carbonation and maintaining a higher pH for a longer time period, increasing so the rebar service life.
Guidelines
National codesBuilding code
A building code, or building control, is a set of rules that specify the minimum acceptable level of safety for constructed objects such as buildings and nonbuilding structures. The main purpose of building codes are to protect public health, safety and general welfare as they relate to the...
also specify minimum cover requirements based on their respective local exposure conditions.
| Country | Concrete Code | Range of Concrete Cover (mm) |
|---|---|---|
| UK | BS:8110 | 25-50 |
| EU | EN 1992 (EC2) | diameter +10 - 55 |
| USA | ACI:318 | 40-50 |
| Australia | AS:3600 | 15-30 |
Paradox
Large cover depths (50–75 mm) are required to protect reinforcement against corrosion in aggressive environments, but thick cover leads to increased crackCrack
Crack may refer to:* Crack cocaine, the freebase form of cocaine that can be smoked* Crack, a fracture or discontinuation in a bodyCrack may also refer to:-Music:* Crack , an album by Z-RO* Cracks , an album by Nabiha...
widths in flexural reinforced concrete members. Large crack-widths (greater than 0.3 mm) permit ingress of moisture and chemical attack to the concrete, resulting in possible corrosion of reinforcement and deterioration of concrete
Concrete degradation
Concrete degradation may have various causes. Concrete can be damaged by fire, aggregate expansion, sea water effects, bacterial corrosion, calcium leaching, physical damage and chemical damage...
. Therefore, thick covers defeat the very purpose for which it is provided. There is a need for judicious balance of cover depth and crack width requirements.
A possible economical solution for this paradox is the placing of a second layer of corrosion-resistant reinforcement like stainless steel
Stainless steel
In metallurgy, stainless steel, also known as inox steel or inox from French "inoxydable", is defined as a steel alloy with a minimum of 10.5 or 11% chromium content by mass....
rebars or mesh
Mesh
Mesh consists of semi-permeable barrier made of connected strands of metal, fiber, or other flexible/ductile material. Mesh is similar to web or net in that it has many attached or woven strands.-Types of mesh:...
es or FRP
FRP
FRP can refer to:* Fibre-reinforced plastic, consists of fiberglass, carbon, aramid, hybrid or other fabric reinforced plastic* Final resting place, as in Cemetery* Franco-Provençal language* Functional reactive programming-Politics:...
rebars in the concrete cover to distribute the cracks.
See also
- Corrosion of reinforcement bars
- Cover meterCover meterA cover meter is an instrument to locate rebars and measure the exact concrete cover. Rebar detectors are less sophisticated devices that can only locate metallic objects below the surface...
- FormworkFormworkFormwork is the term given to either temporary or permanent molds into which concrete or similar materials are poured. In the context of concrete construction, the falsework supports the shuttering moulds.-Formwork and concrete form types:...
- RebarRebarA rebar , also known as reinforcing steel, reinforcement steel, rerod, or a deformed bar, is a common steel bar, and is commonly used as a tensioning device in reinforced concrete and reinforced masonry structures holding the concrete in compression...
- Reinforced concreteReinforced concreteReinforced concrete is concrete in which reinforcement bars , reinforcement grids, plates or fibers have been incorporated to strengthen the concrete in tension. It was invented by French gardener Joseph Monier in 1849 and patented in 1867. The term Ferro Concrete refers only to concrete that is...
- Steel fixerSteel fixerA steel fixer is a tradesman who positions and secures steel reinforcing bars, also known as rebar, and steel mesh used in reinforced concrete on construction projects. The work involves following engineering drawings that detail the type of bar and the spacing used and setting out the work. The...